I tie mine the same way (and Endo does it that way too, as far as I know?
) and I don't have any problems with the koshita either. Although I don't tie it over the ends of my belt, I pull the belt out in the first step and tie below (can't really describe better, sorry...)
I noticed that the koshita seems to slip more when you tuck the hakama under the belt in the beginning, like many people I know.